METHOD FOR CONTROLLING A PROCESS BY MEASUREMENT OF WOOD CHIPS

Abstract: A method to control a manufacturing process for cellulose, pulp or similar by measuring the bulk density of the chips supplied to the process. A representative volume of the supplied chips are sampled and supplied to a measuring method, during which the sampled volume is measured as to the dimensions of the chips contained in the volume, the chip bulk density being possibly calculated by means of the measured values. The thus calculated chip bulk density is used to control the forthcoming chip treatment in the process.

Description: METHOD FOR CONTROLLING A PROCESS BY MEASUREMENT OF WOOD CHIPS. The present invention relates to a control method for defibration process by means of measuring wood chips. For example, in a continous process for digesting cellulos for producing pulp the chips are supplied to the digester b means of chip wheel, such as a feeding-out valve, or by means o a screw. By adjusting the rotation speed of the chip wheel or th screw the parameters for the digesting process are determined an this adjustment is based on the chip bulk density being constant. Should the chip density vary, the digesting process will be ope rating with wrong parameters, thereby influencing the productio yield, the digesting result, the pulp quality, the consumption o chemicals and similar and also influencing the treatments follo wing the digesting process, e.g. bleaching. This may also resul in a negative influence on the environment. To be able to adjust the rotation speed of the chip wheel o the screw laboratory tests are carried out of the chips, durin which a chip sample is taken and the chip bulk density is deter¬ mined by means of any known method, such as volumetric determina¬ tion, weighing and determination of moisture. The chip bulk den¬ sity being determined the received result is used to adjust th rotation speed of the chip wheel or the screw according to th description above and the digesting process or similar is procee ding under the condition that the chip bulk density remains unc hanged. Such laboratory tests are usually performed once or seve¬ ral times a year. As mentioned previously, the chip bulk densit is however seldom constant within one and the same kind of woo during such a long period and will vary substantially down t periods of parts of a day or an hour and the digesting proces will therefore be performed partly under wrong assumptions. The main purpose of the present invention is to propose method to control a cellulose manufacturing process or similar, during which process considerations are made to fluctuations o variations in the chip bulk density in such a way as to conti- nously adapt the process to the changes of the chip bulk densit which may occur during the period of the digesting process. Another purpose is to propose a method, with which samples can be continously taken out from the chips being supplied to the process and to measure from these samples all chip particles as to length, width and thickness and thereby determining the chip bulk density. A further purpose of the invention is to propose a method of the kind mentioned previously, with which the obtained result of the chip bulk density can be presented as a result on a computer screen, representing a control signal for a continous calibration of the incoming chip bulk density in the process. These and further purposes of the invention are achieved by means of a control method for a production process for cellulose, pulp or similar by means of measuring the bulk density of the chips supplied to the process, during which a representative volume of the delivered chips is sampled and is conveyed to a measuring method to measure the selected volume concerning the chip particle dimensions to calculate the chip bulk density by means of the measured values and to use the calculated chip bulk density to control the following chip treatment in the process. According to the invention the calculated bulk density can be sent to a control program for the process as an input signal to adjust the process parameters to the eventually changed values of the chip bulk density. The calculated bulk density can be used to calculate the amount of wood being filled into a container when a certain filling level has been reached, such as in a batch digester, a chip lorry or similar. The calculated bulk density can be used in combination with a weight and volume determination of the chips to detrmine the dry content of the chips, then being sent to a control program to adjust the process parameters to eventually changed values of the chip dry content/moisture. Some of the measured chip dimensions or the chip composition and distribution can be used to control the process. The samples are conveniently sampled with such a interval- time as to maintain a continous measuring method of all the chip particles incorporated in the chip sample. Before measuring the chip particles are separated from each other in the sampled chip volume, e.g. by means of vibration or similar and each chip par¬ ticle in the sampled chip volume is measured as to length, width and thickness. The calculated bulk density is used as a signal to control chip wheel, a screw or a similar device, with which the chips ar supplied to the process. The invention will be described in the following by an embo diment, which however is meant only to illustrate the invention other embodiments and applications being possible within th scope of the claims to follow. As mentioned previously, several known methods do exist t determine the chip bulk density representing time consuming labo ratory tests and thereby providing results which cannot be use to directly control or calibrate the process. In one known method a by volume defined chip mass i weighed, whereafter some of the chips are dried to determine th moisture. Thereafter the bulk density can be determined or calcu lated. In another method the chip diagonal/thickness ratio i determined by screening the chips in two kind of screens, on with which the largest dimension, i.e. the diagonal, is determi ned and one screen to determine the thickness. Thereafter th chip density can be determined. It is obvious that these methods are not conveninet to con tinously determine the chip bulk density. In contrast to these known methods samples are taken in th embodiment according to the invention of the chips being supplie to the process, for example each five minutes or at another con venient interval-time enabling the chips to be sent in a conti nous stream to a measuring method being described later. Thus, the interval-time between sampling is determined mainly by th time needed for the measuring process so that a continous measu ring method of all the chip particles contained in the chip samp le can be maintained. The thus regularly sampled chip samples, being e.g. sampled by separation from a conveyor belt transpor ting the chips to the cellulose process, have then to be separa ted, this being performed by e.g. separating the chip particle from each other by means of vibration and spreading them on bed, such as another conveyer belt, on which they will be trans ported to the measuring method. During the measuring method eac chip particle will be measured as to length, width and thickness, the chip bulk density then possibly to be calculated. The measu ring and the calculation of the chip bulk density can be perfor med by means of some kind of chip analyzer being linked to unit, in which the calculated chip bulk densit value can be use for any further chip treatment in the cellulose process. The cal culated value of the chip bulk density can thus be presented as result on a computer screen for further computation of the chi qualities, such as e.g. the calculation of the chip moisture t be described later. The calculated value of the chip bulk density can even b used as a control signal to continously calibrate the incomin chip bulk density in the process. The calculated value is the used for example to control the device, with which the chips ar supplied to the process, such as a chip measuring wheel, a scre or a similar device, e.g. by regulating the rotating speed of th device. The chip or wood moisture is important to know to optimiz the process as to the product quality, consumption of chemicals, economy, etc. One tries to maintain a certain fluid/wood-ratio i the process. The chip moisture is nowadays measured by laborator means (off line) with variating frequency. Such an analysis, however, is too slow for a direct process control, being perfor¬ med by means of weighing-drying-weighing. There do even exis on-line meters being based on radioactive measuring, radio waves and microwaves or infrared optical systems, these methods, how¬ ever, having all their specific problems and limits. By determing the chip bulk density in accordance with th invention the chip moisture can even be calculated in a simpl way by measuring the weight and the volume without adding an further complicated measuring equipment. Thus, the volume is easily measured by measuring the chip profile height in combina¬ tion with a reading of the belt speed of the belt, with which the chips are transported and the weight is obtained by means of a belt scale for the same belt. The volume can also be determined by a direct measuring in the batch digester, the chip lorry, the feeding-out valve or similar. By means of the values calculated from the chip dimension of the chip bulk density a control parameter is created not bein available previously, with which the process can be controlle for an optimal yield. The measuring procedure can be performe anywhere, where large chip volumes are handled, such as after th cutter or at a chip terminal, a digester or a refiner or somewhe re else, where the best measuring place can be assumed to exis in the plant. As an alternative to control the process by means of th bulk density there is within the scope of the invention even th possibility to use the measured chip dimensions, the chip compo sition or distribution or a combination of these measurements fo said control. The control according to above can of course even be used t control the chip process up-streams the measuring place, such a chip deloading, cutting, chip mixing, screening, etc., the measu ring being preferably arranged at a convenient place in the chi handling process. By sampling a representative test chip volume regularly i this way, being as a whole completely measured as to the indivi dual chip particle dimensions, a safe result will be obtained o the chip bulk density to be used directly or indirectly to cont rol the process the chips are supplied to. A chip analysis com prises all forms of chips, whereby the measuring result become very accurate. It is understood that the method according to the inventio can be used to control other processes than the digester proces described above, such as mechanical or chemical processes or an other process existing in this kind of industry and that the in¬ vention also is intended to comprise any process within the scop of the claims to follow.

Claims: Claims 1. A method to control a manufaturing process for cellulose, pulp or similar by measuring the bulk density of the chips sup¬ plied to the process, characterized in that a representative volume of the supplied chips is sampled and supplied to a measu¬ ring method for measuring the sampled volume as to the dimensions of the chip particles contained in the volume, the chip bulk den¬ sity then being calculable by means of the obtained values and that the thus calculated chip bulk density is used to control the chip treatment in the process, alternatively that any of the mea¬ sured chip dimensions or the chip composition or distribution is used to control the process. 2. A method according to claim 1, characterized in that the calculated chip bulk density or the measured chip dimensions or the chip composition and distribution is used as an input signal to be sent to a control program for the process to adapt the pro¬ cess parameters to eventually changed chip bulk density values. 3. A method according to claim 1, characterized in that the calculated bulk density is used to calculate the amount of wood to be filled into a container, when a certain filling level has been obtained, such as in a batch digester, a chip lorry or simi¬ lar. 4. A method according to claim 1, characterized in that the calculated bulk density is used in combination with a weight and volume determination of the chips to determine the chip moistu¬ re/dry content. 5. A method according to claim 4, characterized in that the chip moisture/dry content as an input signal is sent to a control program for the process to adapt the process parameters to even¬ tually changed chip moisture/dry content values. 6. A method according to claim 1, characterized in that samples are provided with such an interval-time as to maintain continual measuring method of all the chip dimensions incorpora ted in the chip sample. 7. A method according to claims 1-6, characterized in tha the chip particles in the sampled chip volume are separated fro each other before measuring, e.g. by means of vibration or simi lar. 8. A method according to anyone of claims 1-7, characterize in that each chip particle in the sampled chip volume is measure as to length, width and thickness. 9. A method according to anyone of claims 1-7, characterize in that the calculated bulk density or the measured chip dimen¬ sions or the chip composition and distribution is used as a sig¬ nal to control a chip measuring wheel, a screw or a similar devi¬ ce, with which the chips are supplied to the process. 10. A method according to anyone of claims 1-9, characteri¬ zed in that the process consists of a digesting process, a chemi¬ cal or mechanical defibration process or similar. 11. A method according to anyone of claims 1-9, characteri¬ zed in that the calculated bulk density or the measured chip dimensions or the chip composition and distribution is used to control the chip treatment in the process before the measuring place, after the measuring place, or both.

Inventors: BAECKLUND TORBJOERN, BAECKLUND, TORBJOERN
Applicants: BAECKLUND TORBJOERN, IGGESUND TOOLS AB, BAECKLUND, TORBJOERN
Application year: 1994
Main classification: D21C7
IPC codes: D21B1, D21C7, G01N1, G01N9, G01N33